I always want to build that, but end up getting Underforge for Earthshard. This is the XML for that building:

<ImprovementTypeArtDef InternalName="Art_OnyxThrone">
<ImprovementTypeArtSubPack InternalName="Art_OnyxThrone_Kingdom">
<SupportedBuildingStyle>KingdomStyle</SupportedBuildingStyle>
<TileDesign>K_Skath_Pit_01</TileDesign>
<ConstructionTileDesign>K_Skath_Pit_Build_01</ConstructionTileDesign>
</ImprovementTypeArtSubPack>
<ImprovementTypeArtSubPack InternalName="Art_OnyxThrone_Empire">
<SupportedBuildingStyle>EmpireStyle</SupportedBuildingStyle>
<TileDesign>K_Skath_Pit_01</TileDesign>
<ConstructionTileDesign>K_Skath_Pit_Build_01</ConstructionTileDesign>
</ImprovementTypeArtSubPack>
</ImprovementTypeArtDef>

OBVIOUSLY THE WRONG TILE DESIGN. 

Also missing this:

<DrawnIcon></DrawnIcon>
<DrawnIconConstruction></DrawnIconConstruction>

Hope that saves you some time.